injection molding Sink Mark, vents

Sink marks are unintended depressions on the surface of a part that do not mimic the mold steel surface. They are considered a cosmetic defect. Sink marks generally result from an inability to pack out a location within the part and can result from poor process or poor part design (a part having thin and thick regions where the melt must flow from the restrictive thin regions to the thicker region). A sink develops similarly as a void but occurs as a surface defect rather than an internal part defect. To reduce or eliminate sink marks, packing needs to be improved either by process changes or part design changes.

Vents are cuts within the mold steel that allows air to escape.

Air inside of the mold must be allowed to escape so that the plastic can fill the entire space. Without vents, the trapped air will compress as the plastic tries to force the air out of the mold and will cause burning. Generally, a vent should be as large as possible without allowing the plastic to escape.

If the vent is too shallow, the clamping pressure could force it closed. If the vent is too large, plastic could escape and cause flashing. Vents are different sizes based off of what material is being used. Vents should be at the end of flow paths or where flow paths combine. Vents need to be cleaned in order to make sure it does not get clogged and the air can still escape.